Typically, to initially rotate the rotor of a wound rotor induction motor (“WRIM”), a high resistance is inserted into a rotor circuit to produce torque and limit current. As the angular speed of the rotor increases, the resistance is decreased. Typically, the external rotor resistance circuit is electrically connected to the rotor winding via slip rings (also referred to as “collector rings”) and brushes, the brushes applying current to the slip rings to initially energize and rotate the rotor. As the speed approaches rated values, the rotor windings are shorted so that a sufficient magnetic field can be induced into the rotor windings from the stator winding to produce the required torque. However, constantly maintaining brush contact with the slip rings after start-up to achieve the short circuit between the slip rings is inefficient because of the brush wear caused by a friction between the rings and brushes. Additionally, since most brushes are carbon based, carbon dust typically accumulates in the motor from the brush wear.
It is, therefore, desirable to provide systems, methods, and apparatus for shorting slip rings of an induction motor.